The present invention relates to branch pipes mounted to an exhaust manifold of an internal combustion engine, and a method of manufacturing the same.
This type of the exhaust manifold is known as disclosed in the Unexamined Japanese Patent Application Publication No. Hei9-236012.
An example of the exhaust manifold is shown in FIGS. 6 and 7.
An exhaust manifold 1 is made up of a flange 2 fastened to the exhaust side of an engine, four branch pipes 3 coupled to the flange 2, and a manifold container 4 with a branch-pipe mounting part 4a into which those branch pipes 3 are fit in a state that those are arranged in parallel. The fitting part 3a of each branch pipe 3 at which the branch pipe is fit into the branch-pipe mounting part 4a is shaped like a fan in cross section.
An O2 sensor 5 is attached to the manifold container 4 at a position near the base parts of those branch pipes 3.
A manifold converter 6 as a cylindrical container, elliptical in cross section, is coupled to a downstream side of the manifold container 4.
A metal catalyst carrier 7 for purifying exhaust gas is contained in the manifold converter 6.
In the exhaust manifold 1, exhaust gas discharged from the engine is introduced from a plurality of branch pipes 3 into the manifold container 4, and an oxygen concentration in the exhaust gas is measured by the O2 sensor 5 and introduced into the manifold converter 6.
Thereafter, the exhaust gas is purified by the metal catalyst carrier 7, and discharged out through an exhaust gas discharging port 8. Then, it is guided into a muffler (not shown) for deadening the sound of exhausting gases, and then is discharged into the air.
The branch pipes 3 are shaped to have their fitting parts 3a whose cross section is fan in shape.
In the shaping process, the peripheral length adjustment is used. Accordingly, a cross section area of the resultant fitting part is smaller than that of an original pipe 9, as shown in FIG. 8.
Specifically, a cross section area S1 of the fitting part 3a is equal to the result of subtracting cross section areas S2 and S3 of two crescent-like parts from the cross section area S of the original pipe 9.
The reduction of the cross section area will adversely affect the pressure loss. In other words, this possibly leads to the engine output reduction.
Accordingly, an object of the present invention is to provide branch pipes for an exhaust manifold which are able to reduce the pressure loss and a method of manufacturing the same.
The above object is achieved by branch pipes for an exhaust manifold in which a fitting part to be fit into a manifold container comprises a group of pipe members, wherein a cross section of the fitting part of each branch pipe is shaped into a fan-like shape, and the cross sectional area of the branch pipe is substantially equal to that of the branch pipe before the branch pipe is worked.
To achieve the above object, there is also provided a method of manufacturing the exhaust manifold branch pipes as mentioned in aspect 1. The manufacturing method comprises a step of expanding the end of each pipe member, and a step of shaping the expanded end of the pipe member into a fan-like shape in cross section.